With reference to FIG. 1, in conventional transmission control modules for vehicles, a plastic cover 10 is typically bonded to a sealing groove or surface 12 of a cast aluminum base plate or housing 14 using sealant 16. There are frequently reports of leakage into housing due to non-sticking of the sealant to the surface of the aluminum housing. FIG. 1 shows the cover opened with respect to the housing 14. As seen, while the sealant 16 bonds cohesively to the plastic cover 10, the sealant 16 tends to peel-off from the sealing surface 12 of the aluminum housing 14.
Analysis indicates that the surface of the cast aluminum is contaminated with a mold release agent and/or thermally degraded byproducts from the molding process. The contaminants can cause the loss of the adhesion. Tests appear to further indicate that the contaminants either combine chemically to the surface of cast aluminum, or penetrate into the top layer of the cast aluminum during the cast molding process that has the alloy melting temperature of over 600C. A general soap wash from the casting supplier, or even isopropyl alcohol (IPA) wiping is not effective to clean out the residual release agent. More aggressive surface treatment methods were used to see how they could effectively clean out the contaminants from the surface of the cast aluminum groove. The results show that even ultrasonic cleaning, using aggressive solvents (Dichloromethane 10 min+Hexane 10 min), is still not very effective. Furthermore, regular plasma or 5× stronger plasma does not provide a clean enough surface to generate cohesive adhesion.
Testing has also shown that cohesive bonding to the surface of the aluminum can be achieved by an extremely slow speed plasma, or by surface sanding. However, the slow plasma method (1 mm/sec) is too slow and not feasible for mass production. Using higher speed was found to not be effective. Sanding (or shot peening, or sand blasting) the surface will eventually accumulate the contaminant on the surface of the sand or metal beads, which could cross-contaminant the surface of the aluminum after using this method over time.
Thus, there is a need to remove a top layer of a surface of cast metal by laser trimming to clean the surface and ensure adhesion of a sealant to the cleaned surface.